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Coordinating Neuromuscular Electrical Stimulation with Robotic Treadmill Training to Promote Rehabilitation of Independent Walking

Spinal cord injury patients could benefit in a multitude of ways from regaining the ability to walk.  Not only would they have mobility to enable them to get from one place to another, but their systemic health would benefit from the increased blood circulation and cardiovascular activity.  Although physical therapists can assist patients to perform walking movements on a treadmill, not only is this a costly therapy to continue in the long run, the movements the patients legs go through are passive, not generated primarily by the patients' own control.  Neuromuscular electrical stimulation (NMES)  can be added to such treadmill walking to activate muscles at certain times which would allow them to walk better with less assistance from a physical therapist.  This approach has not been used much yet but has shown positive results over treadmill training or NMES alone.  Thus far, the peripheral nerves have been stimulated at pre-set times relative to the initiation of a gait cycle. As far as the literature shows, this therapy was designed to serve immediate needs, analogous to a walking crutch, in that once the stimulation is taken away, the artificially triggered muscle activations which aid with walking are also expected to disappear. Additionally, these pre-programmed NMES systems induce fatigue quickly and result in an unnatural gait.

To address the aforementioned limitations of present NMES technology, we are developing a new NMES system which times stimulation to robotically controlled hindlimb position in spinal cord injured rodent animals during stepping. Our long-term aim in combining NMES with robotically controlled treadmill training (RTT) is to enhance activation of the spinal cord circuitry that controls gait. The rationale is that by stimulating neural pathways between the spinal cord and muscles at appropriate times, the therapy will reinforce spinal circuits which generate stepping.

The proposed NMES therapy will apply two different stimulation patterns: the “motor pattern” to elicit motor activity when mechanosensor-induced afferent activity has been predicted to have occurred (Fig. 1B, dashed --- trace), and the “sensory pattern” to induce motor activity by stimulating sensory axons which trigger walking-related spinal circuits (Fig. 1C, dashed --- trace). This approach is based on evidence of two principles: 1) that appropriately timed afferent activity during stepping influences and is important for spinal reorganization; and 2) that invoking activity-dependent plasticity also requires that the hindlimb stepping that we desire to reinforce is coincident with the stimulation therapy. The motor pattern is hypothesized to reinforce spinal circuitry which controls walking by inducing synaptic potentiation (Fig. 1B); the sensory pattern is hypothesized to elicit centrally controlled walking behavior (Fig. 1C); i.e., walking controlled by the spinal cord as would more naturally be the case in non-injured individuals. Thus, stimulation patterns will need to be designed which preferentially evoke sensory or motor nerves. Different combinations of pulse width and frequency have been found to differentially activate motor and sensory nerves, but these findings have been used to study electrophysiological behavior rather than to develop novel therapeutics for SCI.